Commercial service of a wideband code division multiple access (W-CDMA) system among so-called third-generation communication systems has been offered in Japan since 2001. In addition, high speed downlink packet access (HSDPA) service for achieving higher-speed data transmission using a downlink has been offered by adding a channel for packet transmission (high speed-downlink shared channel (HS-DSCH)) to the downlink (dedicated data channel, dedicated control channel). Further, in order to increase the speed of data transmission in an uplink direction, service of a high speed uplink packet access (HSUPA) system has been offered. W-CDMA is a communication system defined by the 3rd generation partnership project (3GPP) that is the standard organization regarding the mobile communication system, where the specifications of Release 10 version are produced.
Further, 3GPP is studying new communication systems referred to as long term evolution (LTE) regarding radio areas and system architecture evolution (SAE) regarding the overall system configuration including a core network (hereinafter, merely referred to as “network” as well) as communication systems independent of W-CDMA. This communication system is also referred to as 3.9 generation (3.9 G) system.
In the LTE, an access scheme, a radio channel configuration, and a protocol are totally different from those of the W-CDMA (HSDPA/HSUPA). For example, as to the access scheme, code division multiple access is used in the W-CDMA, while in the LTE, orthogonal frequency division multiplexing (OFDM) is used in a downlink direction and single career frequency division multiple access (SC-FDMA) is used in an uplink direction. In addition, the bandwidth is 5 MHz in the W-CDMA, while in the LTE, the bandwidth can be selected from 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz per base station. Further, differently from the W-CDMA, circuit switching is not provided but a packet communication system is only provided in the LTE.
In the LTE, a communication system is configured with a new core network different from the general packet radio service (GPRS) being the core network of the W-CDMA, and thus, the radio access network of the LTE is defined as a radio access network independent of the W-CDMA network.
Therefore, for differentiation from the W-CDMA communication system, a radio access network is referred to as an evolved universal terrestrial radio access (E-UTRAN) in the LTE communication system. The base station that communicates with a mobile terminal (user equipment (UE)) being a communication terminal device is referred to as an E-UTRAN NodeB (eNB). The radio network controller that exchanges control data and user data with a plurality of base stations is also referred to as an evolved packet core (EPC) or an access gateway (aGW).
Non-Patent Document 1 (Chapter 4) describes the current decisions by 3GPP regarding an overall architecture in the LTE system. The overall architecture will be described with reference to FIG. 1. FIG. 1 is a diagram illustrating the configuration of the LTE communication system. With reference to FIG. 1, the E-UTRAN is composed of one or a plurality of base stations 102, provided that a control protocol for a user equipment 101 such as a radio resource control (RRC), and user planes such as a packet data convergence protocol (PDCP), radio link control (RLC), medium access control (MAC) and physical layer (PHY) are terminated in the base station 102.
The base stations 102 perform scheduling and transmission of a paging signal (also referred to as paging messages) notified from a mobility management entity (MME) 103. The base stations 102 are connected to each other by means of an X2 interface. In addition, the base stations 102 are connected to an evolved packet core (EPC) 105 by means of an S1 interface. More specifically, the base station 102 is connected to the mobility management entity (MME) 103 of the EPC 105 by means of an S1_MME interface and connected to a serving gateway (S-GW) 104 of the EPC 105 by means of an S1_U interface.
The MME 103 distributes the paging signal to a plurality of or a single base station 102. In addition, the MME 103 performs mobility control of an idle state. When the user equipment is in the idle state and an active state, the MME 103 manages a list of tracking areas.
The S-GW 104 transmits/receives user data to/from one or a plurality of base stations 102. The S-GW 104 serves as a local mobility anchor point in handover between base stations. Moreover, a PDN gateway (P-GW, which is not shown here) is provided in the EPC 105. The P-GW performs per-user packet filtering and UE-ID address allocation.
The control protocol RRC between the user equipment 101 and the base station 102 performs broadcast, paging, RRC connection management, and the like. The states of the base station and the user equipment in RRC are classified into RRC_IDLE and RRC_CONNECTED. In RRC_IDLE, public land mobile network (PLMN) selection, system information (SI) broadcast, paging, cell re-selection, mobility, and the like are performed. In RRC_CONNECTED, the user equipment has RRC connection and is capable of transmitting/receiving data to/from a network. In RRC_CONNECTED, for example, handover (HO) and measurement of a neighbour cell are performed.
The decisions by 3GPP regarding the frame configuration in the LTE system described in Non-Patent Document 1 (Chapter 5) will be described with reference to FIG. 2. FIG. 2 is a diagram illustrating the configuration of a radio frame used in the LTE communication system. With reference to FIG. 2, one radio frame is 10 ms. The radio frame is divided into ten equally sized subframes. The subframe is divided into two equally sized slots. The first and sixth subframes contain a downlink synchronization signal (SS) per each radio frame. The synchronization signals are classified into a primary synchronization signal (P-SS) and a secondary synchronization signal (S-SS).
Non-Patent Document 1 (Chapter 5) describes the decisions by 3GPP regarding the channel configuration in the LTE system. It is assumed that the same channel configuration is used in a closed subscriber group (CSG) cell as that of a non-CSG cell. Physical channels will be described with reference to FIG. 3. FIG. 3 is a diagram illustrating physical channels used in the LTE communication system.
With reference to FIG. 3, a physical broadcast channel (PBCH) 401 is a channel for downlink transmission from the base station (eNB) 102 to the user equipment (UE) 101. A BCH transport block is mapped to four subframes within a 40 ms interval. There is no explicit signaling indicating 40 ms timing.
A physical control format indicator channel (PCFICH) 402 is a channel for downlink transmission from the base station 102 to the user equipment 101. The PCFICH notifies the number of OFDM symbols used for PDCCHs from the base station 102 to the user equipment 101. The PCFICH is transmitted in each subframe.
A physical downlink control channel (PDCCH) 403 is a channel for downlink transmission from the base station 102 to the user equipment 101. The PDCCH notifies the resource allocation information for a downlink shared channel (DL-SCH) being one of the transport channels shown in FIG. 4 described below, resource allocation information for a paging channel (PCH) being one of the transport channels shown in FIG. 4, and hybrid automatic repeat request (HARQ) information related to DL-SCH. The PDCCH carries an uplink scheduling grant. The PDCCH carries acknowledgement (Ack)/negative acknowledgement (Nack) that is a response signal to uplink transmission. The PDCCH is referred to as an L1/L2 control signal as well.
A physical downlink shared channel (PDSCH) 404 is a channel for downlink transmission from the base station 102 to the user equipment 101. A downlink shared channel (DL-SCH) that is a transport channel and a PCH that is a transport channel are mapped to the PDSCH.
A physical multicast channel (PMCH) 405 is a channel for downlink transmission from the base station 102 to the user equipment 101. A multicast channel (MCH) that is a transport channel is mapped to the PMCH.
A physical uplink control channel (PUCCH) 406 is a channel for uplink transmission from the user equipment 101 to the base station 102. The PUCCH carries Ack/Nack that is a response signal to downlink transmission. The PUCCH carries a channel quality indicator (CQI) report. The CQI is quality information indicating the quality of received data or channel quality. In addition, the PUCCH carries a scheduling request (SR).
A physical uplink shared channel (PUSCH) 407 is a channel for uplink transmission from the user equipment 101 to the base station 102. An uplink shared channel (UL-SCH) that is one of the transport channels shown in FIG. 4 is mapped to the PUSCH.
A physical hybrid ARQ indicator channel (PHICH) 408 is a channel for downlink transmission from the base station 102 to the user equipment 101. The PHICH carries Ack/Nack that is a response signal to uplink transmission. A physical random access channel (PRACH) 409 is a channel for uplink transmission from the user equipment 101 to the base station 102. The PRACH carries a random access preamble.
A downlink reference signal (RS) is a known symbol in the LTE communication system. The following five types of downlink reference signals are defined: cell-specific reference signals (CRS), MBSFN reference signals, data demodulation reference signal (DM-RS) being UE-specific reference signals, positioning reference signals (PRS), and channel-state information reference signals (CSI-RS). The physical layer measurement objects of a user equipment include reference signal received power (RSRP).
The transport channels described in Non-Patent Document 1 (Chapter 5) will be described with reference to FIG. 4. FIG. 4 is a diagram illustrating transport channels used in the LTE communication system. FIG. 4(A) shows mapping between downlink transport channels and downlink physical channels. FIG. 4(B) shows mapping between uplink transport channels and uplink physical channels.
A broadcast channel (BCH) among the downlink transport channels shown in FIG. 4(A) is broadcast to the entire coverage of a base station (cell). The BCH is mapped to the physical broadcast channel (PBCH).
Retransmission control according to a hybrid ARQ (HARQ) is applied to a downlink shared channel (DL-SCH). The DL-SCH enables broadcast to the entire coverage of the base station (cell). The DL-SCH supports dynamic or semi-static resource allocation. The semi-static resource allocation is also referred to as persistent scheduling. The DL-SCH supports discontinuous reception (DRX) of a user equipment for enabling the user equipment to save power. The DL-SCH is mapped to the physical downlink shared channel (PDSCH).
The paging channel (PCH) supports DRX of the user equipment for enabling the user equipment to save power. The PCH is required to broadcast to the entire coverage of the base station (cell). The PCH is mapped to physical resources such as the physical downlink shared channel (PDSCH) that can be used dynamically for traffic.
The multicast channel (MCH) is used for broadcast to the entire coverage of the base station (cell). The MCH supports SFN combining of MBMS services (MTCH and MCCH) in multi-cell transmission. The MCH supports semi-static resource allocation. The MCH is mapped to the PMCH.
Retransmission control according to a hybrid ARQ (HARQ) is applied to an uplink shared channel (UL-SCH) among the uplink transport channels shown in FIG. 4(B). The UL-SCH supports dynamic or semi-static resource allocation. The UL-SCH is mapped to the physical uplink shared channel (PUSCH).
A random access channel (RACH) is limited to control information. The RACH involves a collision risk. The RACH is mapped to the physical random access channel (PRACH).
The logical channels described in Non-Patent Document 1 (Chapter 6) will be described with reference to FIG. 5. FIG. 5 is a diagram illustrating logical channels used in an LTE communication system. FIG. 5(A) shows mapping between downlink logical channels and downlink transport channels. FIG. 5(B) shows mapping between uplink logical channels and uplink transport channels.
A broadcast control channel (BCCH) is a downlink channel for broadcast system control information. The BCCH that is a logical channel is mapped to the broadcast channel (BCH) or downlink shared channel (DL-SCH) that is a transport channel.
A paging control channel (PCCH) is a downlink channel for transmitting paging information and system information change notifications. The PCCH is used when the network does not know the cell location of a user equipment. The PCCH that is a logical channel is mapped to the paging channel (PCH) that is a transport channel.
A common control channel (CCCH) is a channel for transmission control information between user equipments and a base station. The CCCH is used in a case where the user equipments have no RRC connection with the network. In a downlink direction, the CCCH is mapped to the downlink shared channel (DL-SCH) that is a transport channel. In an uplink direction, the CCCH is mapped to the uplink shared channel (UL-SCH) that is a transport channel.
A multicast control channel (MCCH) is a downlink channel for point-to-multipoint transmission. The MCCH is used for transmission of MBMS control information for one or several MTCHs from a network to a user equipment. The MCCH is used only by a user equipment during reception of the MBMS. The MCCH is mapped to the multicast channel (MCH) that is a transport channel.
A dedicated control channel (DCCH) is a point-to-point channel that transmits dedicated control information between a user equipment and a network. The DCCH is used if the user equipment has an RRC connection. The DCCH is mapped to the uplink shared channel (UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) in downlink.
A dedicated traffic channel (DTCH) is a point-to-point communication channel for transmission of user information to a dedicated user equipment. The DTCH exists in uplink as well as downlink. The DTCH is mapped to the uplink shared channel (UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) in downlink.
A multicast traffic channel (MTCH) is a downlink channel for traffic data transmission from a network to a user equipment. The MTCH is a channel used only by a user equipment during reception of the MBMS. The MTCH is mapped to the multicast channel (MCH).
CGI represents a cell global identification. ECGI represents an E-UTRAN cell global identification. A closed subscriber group (CSG) cell is introduced in the LTE, and the long term evolution advanced (LTE-A) and universal mobile telecommunication system (UNITS) described below. The CSG will be described below (see Chapter 3.1 of Non-Patent Document 2).
The closed subscriber group (CSG) cell is a cell in which subscribers who are allowed to use are specified by an operator (also referred to as a “cell for specific subscribers”). The specified subscribers are allowed to access one or more cells of a public land mobile network (PLMN). One or more cells in which the specified subscribers are allowed access are referred to as “CSG cell(s).” Note that access is limited in the PLMN.
The CSG cell is part of the PLMN that broadcasts a specific CSG identity (CSG ID; CSG-ID) and broadcasts “TRUE” in a CSG indication. The authorized members of the subscriber group who have registered in advance access the CSG cells using the CSG-ID that is the access permission information.
The CSG-ID is broadcast by the CSG cell or cells. A plurality of CSG-IDs exist in the LTE communication system. The CSG-IDs are used by user equipments (UEs) for making access from CSG-related members easier.
The locations of user equipments are tracked based on an area composed of one or more cells. The locations are tracked for enabling tracking of the locations of user equipments and calling user equipments, in other words, incoming calling to user equipments even in an idle state. An area for tracking locations of user equipments is referred to as a tracking area.
The CSG whitelist is a list that may be stored in a universal subscriber identity module (USIM) in which all CSG IDs of the CSG cells to which the subscribers belong are recorded. The CSG whitelist may be merely referred to as a whitelist or an allowed CSG list as well. As to the access of user equipments through a CSG cell, the MME performs access control (see Chapter 4.3.1.2 of Non-Patent Document 3). Specific examples of the access of user equipments include attach, combined attach, detach, service request, and a tracking area update procedure (see Chapter 4.3.1.2 of Non-Patent Document 3).
The service types of a user equipment in an idle state will be described below (see Chapter 4.3 of Non-Patent Document 2). The service types of user equipments in an idle state include a limited service, standard service (normal service), and operator service. The limited service includes emergency calls, earthquake and tsunami warning system (ETWS), and commercial mobile alert system (CMAS) on an acceptable cell described below. The standard service (also referred to as normal service) is a public service on a suitable cell described below. The operator service includes a service for operators only on a reserved cell described below.
A “suitable cell” will be described below. The “suitable cell” is a cell on which a UE may camp to obtain normal service. Such a cell shall fulfill the following conditions (1) and (2).
(1) The cell is part of the selected PLMN or the registered PLMN, or part of the PLMN of an “equivalent PLMN list.”
(2) According to the latest information provided by a non-access stratum (NAS), the cell shall further fulfill the following conditions (a) to (d):
(a) the cell is not a barred cell;
(b) the cell is part of a tracking area (TA), not part of the list of “forbidden LAs for roaming,” where the cell needs to fulfill (1) above;
(c) the cell shall fulfill the cell selection criteria; and
(d) for a cell specified as CSG cell by system information (S1), the CSG-1D is part of a “CSG whitelist” of the UE, that is, is contained in the “CSG whitelist” of the UE.
An “acceptable cell” will be described below. The “acceptable cell” is a cell on which a UE may camp to obtain limited service. Such a cell shall fulfill the all following requirements (1) and (2).
(1) The cell is not a prohibited cell (also referred to as a “barred cell”).
(2) The cell fulfills the cell selection criteria.
“Barred cell” is indicated in the system information. “Reserved cell” is indicated in the system information.
“Camping on a cell” represents the state where a UE has completed the cell selection/cell reselection process and the UE has selected a cell for monitoring the system information and paging information. The cell on which the UE camps may be referred to as a “serving cell.”
3GPP is studying base stations referred to as Home-NodeB (Home-NB; HNB) and Home-eNodeB (Home-eNB; HeNB). HNB/HeNB is a base station for, for example, household, corporation, or commercial access service in UTRAN/E-UTRAN.
In a typical communication system supporting the movement on the ground, a large-scale base station configuring a relatively large large-scale cell configures a relatively wide service area. A small-scale base station configuring a small-scale cell having a relatively small service area is installed in a specific place in the wide service area, such as a house to process the communication of a user in the house by the small-scale base station, thereby reducing the processing load of the large-scale base station and improving the communication quality of the user in the house.
Non-Patent Document 4 discloses three different modes of the access to the HeNB and HNB. Specifically, an open access mode, a closed access mode, and a hybrid access mode are disclosed.
The respective modes have the following characteristics. In the open access mode, the HeNB and HNB are operated as a normal cell of a normal operator. In the closed access mode, the HeNB and HNB are operated as a CSG cell. The CSG cell in the closed access mode is a CSG cell where only CSG members are allowed access. In the hybrid access mode, the HeNB and HNB are operated as CSG cells where non-CSG members are allowed access at the same time. In other words, a cell in the hybrid access mode (also referred to as a hybrid cell) is a cell that supports both the open access mode and the closed access mode.
In 3GPP, among all physical cell identities (PCIs), there is a range of PCIs reserved by the network for use by CSG cells (see Chapter 10.5.1.1 of Non-Patent Document 1). Division of the PCI range is also referred to as PCI split. The PCI split information is broadcast in the system information from a base station to user equipments being served thereby. A user equipment served by a base station means a user equipment that takes the base station as a serving cell.
Non-Patent Document 5 discloses the basic operation of a user equipment using PCI split. The user equipment that does not have the PCI split information needs to perform cell search using all PCIs, for example, using all 504 codes. On the other hand, the user equipment that has the PCI split information is capable of performing cell search using the PCI split information.
The base station has two operation modes, namely, a normal mode and an energy saving mode. In the normal mode, a base station performs a transmission operation for a downlink transmission signal to be transmitted to a user equipment and a reception operation for an uplink transmission signal transmitted from the user equipment. The base station accordingly provides services to user equipments being served thereby. In the energy saving mode, the base station stops at least the transmission operation for a downlink transmission signal, thereby stopping the provision of services to the user equipments being served thereby.
The base station configures one or a plurality of cells. In the case where the base station configures a plurality of cells, the base station is configured to be switchable between the normal mode and the energy saving mode per cell.
In the LTE communication system, a relatively small cell (hereinafter, also referred to as a “hotspot cell”) may be deployed in a cell (hereinafter, also referred to as a “coverage cell”) configuring a basic service area to locally increase service capacity. The coverage cell is a large-scale cell and has a relatively large coverage. The hotspot cell is a small-scale cell and has a relatively small coverage.
For relatively high traffic in the communication system, a coverage cell and a hotspot cell are both operated in the normal mode. If the coverage cell alone can allocate service capacity due to decreased traffic, the hotspot cell may shift to the energy saving mode. When the traffic of the coverage cell increases, the base station that configures a coverage cell (hereinafter, also referred to as a “coverage cell base station”) shifts any of the base stations that configure a hotspot cell (hereinafter, also referred to as “hotspot cell base stations”) from the energy saving mode to the normal mode.
Non-Patent Document 6 discloses the method of shifting the base station, which has entered the energy saving mode and stopped the provision of services to user equipments, to the normal mode.
In the method disclosed in Non-Patent Document 6, for example, when detecting high traffic, the coverage cell base station instructs a plurality of hotspot cell base stations to cancel the energy saving mode and to measure interference. Each hotspot cell base station cancels the energy saving mode and measures interference, and then reports the interference to the coverage cell base station.
The coverage cell base station that has reported interference determines a target hotspot cell base station whose operation is to be restarted from a plurality of hotspot cell base stations that have reported interference, and instructs the determined hotspot cell base station to start issuing a pilot signal.
The hotspot cell base station that has been instructed to start issuing a pilot signal issues a pilot signal. Upon detection of the pilot signal from the hotspot cell base station, the UE reports the detection of the pilot signal to the coverage cell base station. The coverage cell base station requests the MME to perform handover and, if the MME permits it, instructs the UE to perform handover. The UE performs handover from the coverage cell base station to the hotspot cell base station.